It is known that in conventional filter elements different filter material is combined for generating a filter behavior that is optimal for the particles to be filtered out and the fluid passing through. For example, DE 44 43 156 A1 in itself discloses the use of a meltblown as a filter medium in a gas stream together with a support material that serves exclusively a stabilization purpose.
Moreover, WO 96/34673 discloses in connection with a hollow-cylindrical filter element the arrangement of several filter layers of a meltblown on a support layer. The layers then form together an exchangeable filter element that is insertable into a filter arrangement.
U.S. Pat. No. 5,496,627 and WO 95/17946 disclose sequentially arranged nonwoven filter elements of synthetic fibers with stepped filter fineness wherein here the filter fineness of the filter layers increases in the flow direction.
Moreover, U.S. Pat. No. 5,427,597 and WO 96/34673 disclose that several filter layers or only a single filter layer of a nonwoven produced by a meltblowing method are arranged on a support layer that serves substantially as a stabilizing means. The filtering action of the support layer is neglectable in comparison to the other layers in this connection.
Finally, U.S. 2007/0289920 A1 discloses a filter arrangement with a filter medium that contains a plurality of layers of a spunbond nonwoven with continuous fibers. In addition, at least one inwardly positioned layer of meltblown fibers is provided between the outer spunbond layers.
The invention has the object to provide a multi-layer fully synthetic filter medium with a gradient structure that in addition to a very high degree of separation also is distinguished by very high dirt absorption and a minimal constructive size.